Hello! I am relatively new to
40k after a long period of not playing and I am learning the ropes of my guard army and after a few games I learnt a few things already but theory always seems easier, well for me anyway.
First thing is to know thy enemy, which has been said already. For example, orks are
cc oriented and hard units such as ork bikes and battlewagons, which should be taken care of first for example. Stop the assualt in no man's land and let them come towards you without their transports.
With eldar, yes, shoot them up before they get to you and as I play guard try to slow their advance with a tarpit unit, in my case a power blob so you give time for your other units to shoot. Of course I am saying this from a shooty army angle and not a
CC one but the principle might be the same?
Against armies that have deepstrike ability I always try to remember to hold at least half my army in reserve and perhaps even a tank or two, quite a surprise for a group of terminators to suddenly see a demolisher cannon or medusa at close range after they deepstriked
For me it boils down to target practice, shoot at what can hurt you the most and concentrate on it till it is dead, rinse and repeat, well in theory anyway...
Also most importantly that it is a game and as long as you have fun then great, of course learn from your games and there is always next time.